A number of different oligomerisation technologies are known to produce α-olefins. Some of these processes, including the Shell Higher Olefins Process and Ziegler-type technologies, have been summarized in WO 04/056479 A1. The same document also discloses that the prior art (e.g. WO 03/053891 and WO 02/04119) teaches that chromium based catalysts containing heteroaromatic ligands with both phosphorus and nitrogen heteroatoms, selectively catalyse the trimerisation of ethylene to 1-hexene.
Processes wherein transition metals and heteroaromatic ligands are combined to form catalysts for trimerisation, tetramerisation, oligomerisation and polymerisation of olefinic compounds have also been described in different patent applications such as WO 03/053890 A1; WO 03/053891; WO 04/056479 A1; WO 04/056477 A1; WO 04/056480 A1; WO 04/056478 A1; WO 05/123884 A2; WO 05/123633 A1 and U.S. Pat. No. 7,285,607.
The catalysts utilized in the abovementioned trimerisation, tetramerisation, oligomerisation or polymerisation processes all include one or more activators to activate the catalyst. Such an activator is a compound that generates an active catalyst when the activator is combined with the catalyst.
Suitable activators include organoaluminium compounds, organoboron compounds, organic salts, such as methyl lithium and methyl magnesium bromide, inorganic acids and salts, such as tetrafluoroboric acid etherate, silver tetrafluoroborate, sodium hexafluoroantimonate and the like.
A common catalyst activator used in combination with Cr based catalysts for oligomerisation of olefinic compounds is alkylaluminoxane, particularly methylaluminoxane (MAO). It is well known that MAO includes significant quantities of alkylaluminium in the form of trimethylaluminium (TMA), and in effect the catalyst activator is a combination of TMA and MAO. The MAO may also be replaced with modified MAO (MMAO), which may contain free trialkylaluminium in the form of TMA and heavier trialkylaluminiums. The use of organoboron compounds as catalyst activators is also known.
Activators containing aluminium compounds are costly to the effect that it impacts significantly on process economics of olefin oligomerisation technologies that utilize this class of activators. For this reason, it is desirable to run commercial oligomerisation processes at low activator concentrations. However, in the case where an aluminium-containing compound was used as an activator for transition metal based oligomerisation catalysts, it was found that at conditions of low starting aluminium concentrations (e.g. <6 mmol/l), low reaction rates and high levels of unwanted solid formation (polyethylene (PE) and waxes) resulted when ethylene was oligomerised.
Reduction in the formation of polymer as a by-product in Cr-based ethylene oligomerisation (both tri- and tetramerisation) processes remains an ongoing challenge, as polymer fouling reduces plant run time and necessitates shut-downs due to blockages.
The inventors of the present invention have found that reductions in polymer formation levels can be achieved in the chromium catalysed olefin oligomerisation processes by the incorporation of a zinc compound, in particular dialkyl zinc, in the catalyst system in the manner described below.
The use of a zinc compound in olefin oligomerisation is not unknown, but it has not been disclosed previously that it may be used in the manner of the present invention to achieve a reduction in polymer formation as herein disclosed. Thus the use of chain transfer reagents within the field of polymerisation has long been known. For example the use of triethylaluminium in combination with various ‘chain growth’ type polymerisation catalysts (e.g. 1,3,7-triazacyclonanone) has been previously studied.
WO 2008/085659 and US 2008/0058486 both disclose the use of various activators for oligomerisation catalyst systems. Both contain the statement that “(o)ther general activators or compounds useful in an oligomerization reaction may be used. These compounds may be activators in some contexts, but may also serve other functions in the reaction system, such as alkylating a metal center or scavenging impurities. These compounds are within the general definition of “activator,” but are not considered herein to be ion-forming activators.” Both these publications broadly identify these “other” components as Group 13 reagents, divalent metal reagents, and alkali metal reagents and names diethyl zinc as one of a large group of reagents said to be “useful as activators for the catalyst compounds” with which the publications are concerned. However, neither of these publications exemplify such use of these “other” components and in particular of diethyl zinc, nor do they disclose any benefit derived from the use of diethyl zinc, or any effect that such use of diethyl zinc might have on the extent of polymer formation in chromium catalysed olefin oligomerisation.